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The Coolant Recovery Bottle: Unsung Hero of the Engine Room

Here's how to make sure your coolant recovery bottle is properly installed and in good working order.

A client once sent me an email regarding her engine. She was in the midst of having a new vessel she and her husband had recently acquired, brought up to their exact, offshore cruising standards. She’s the “ship’s engineer” and thus oversees most of the systems, electrical, hull and deck related projects, service and repairs, and is among the most knowledgeable boat owners I’ve ever encountered. When I say knowledgeable, I don’t mean in the sense of all things boat, I mean she makes it her business to know her vessels really well. In an email from her husband one time, he said, “Linda says hi, she’s changing the hydraulic stabilizer fluid.” Thus, working with her is a pleasure; she’s the poster child for my "know your boat’s systems really well" mantra.

The email she sent on this occasion had to do with the vessel’s engines’ closed, pressurized cooling system, and specifically, the pressure cap and recovery bottle. Most boat owners, even those with limited technical abilities, know that the cooling system on their engine and generator operates under some level of pressure. But have you ever thought about why that’s the case? In some ways it seems counterintuitive, pressurizing this system makes leaks more likely, right? That is true; however, it’s worth the risk. The query surrounded advice she’d received from a mechanic to modify the cooling systems pressure relief cap. The cap itself is worthy of some discussion—it’s a rather complex piece of machinery that typically incorporates two calibrated springs as well as two gaskets and a check valve. The primary spring controls the pressure at which relief will occur (i.e. when coolant will be allowed to leave the expansion tank, the assembly on which the cap is typically mounted.) As the engine heats up so, too, does the coolant; the latter expands and must be given a route to do so—typically into a recovery bottle, but sometimes into the bilge under the engine.

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Cooling system caps are often rated for pressure of somewhere between six and 15 pounds per square inch and this rating is nearly always embossed on the cap. The pressure rating is designated by the engine manufacturer and replacement caps must never deviate from this specification. The cooling system’s gaskets, pump seals and hoses are specifically designed to operate at a given pressure, exceeding it can lead to failures of these components and leaks. The cap gaskets serve several purposes: They keep coolant from leaking out of the system, they maintain pressure, and if equipped with one, they allow coolant to be drawn back into the system from the coolant recovery bottle.

There are several reasons why a cooling system benefits from pressurization. For the purposes of this discussion, the primary reason for operating under pressure is to increase the boiling point of the coolant and thus the efficiency of the cooling system. For every pound of pressure that the system operates under, the boiling point (of water) is raised three degrees Fahrenheit. Therefore, a 15-pound pressure cap raises the boiling point 45 degrees, affording an additional margin against boiling and overheating.

The coolant recovery bottle’s primary purpose is to capture expanding coolant, as well as keeping the cooling system from ingesting air, which could lead to pump cavitation and overheating. Among the recovery bottle’s most valuable traits, however, is its ability to alert the user to trouble within the system. If, for instance, the level drops steadily over time, it’s an indication that coolant is leaking from the system, and that it’s being “consumed” from the bottle. You should never have to add coolant to the system once it’s undergone the first few operating cycles.

A Valuable Tool

It comes as a surprise to many users that the bottle’s coolant level should fluctuate during an operating cycle, which is from cold start-up, to full operating temperature, and back to cold. That is, the level of coolant in the recovery bottle should rise as the engine reaches operating temperature. Once the engine is shut down and cools off, it should drop back to its original starting level. Mark the cold level, so you will immediately note an irregularity. If there is no movement, or if it drops below the starting level, it’s an indication of a problem. No movement often indicates a problem with the cap’s filler neck port, which is connected to the bottle via a hose; it sometimes becomes clogged with debris, or it may be cracked, or the hose itself may be crushed or breached. 

In other cases, a lack of intra-heating cycle movement may indicate a defective pressure cap. Its internal mechanism is somewhat delicate and can cease to retain a vacuum, which is necessary to draw coolant from the bottle to the expansion tank during the cool-off period. When the system is cool, remove and inspect the cap to ensure its gaskets are clean and free of tears or other damage. Inspect the filler neck as well for dents and corrosion, and to ensure the overflow port is clear. Finally, an accumulation of black gritty or oily flotsam in the coolant recovery bottle—a dire warning sign should not be ignored—can be an indication of a head gasket or oil cooler leak.


Coolant recovery bottle placement is frequently a source of confusion for boatbuilders, mechanics, and boat owners. Because the bottle operates on the principles of pressure and vacuum, gravity plays little or no role in its proper operation. Therefore, contrary to popular belief, the bottle need not be placed above the engine. If it is, opening the pressure cap for inspection purposes virtually guarantees that coolant will overflow from the expansion tank filler neck. 

Ideally, the bottle should be placed so that the cold coolant fill mark is roughly level with the height of the pressure cap. The hose should be routed in such a way to avoid blocking a thoroughfare and prevent damage from chafe or being stepped on. Finally, while it is ubiquitous, clear PVC hose should not be used to connect the recovery bottle to the expansion tank. This hose lacks the chafe, temperature, crush, and kink resistance necessary for this vital application. Instead, a more rugged, coolant resistant hose such as Type B fuel line should be used.

Ultimately, my advice to the client was to avoid making any modifications to the cooling system pressure cap. Pressure caps have, over the years, been virtually perfected, and as such, the only input they require from users is periodic inspection and replacement when they’ve reached the end of their useful life.

Your coolant recovery bottle is the window to your engine’s cooling system. Make sure yours is properly installed and in good working order, and it may very well alert you to problems long before they otherwise manifest themselves.